1. Field of the Invention
The invention relates to a method and an apparatus for electrochemically roughening a substrate for light-sensitive layers. The surface of the substrate is roughened electrochemically, or mechanically and subsequently electrochemically in an aqueous electrolyte bath by the application of an alternating or three-phase current to electrodes opposite the substrate, while the substrate is passed continuously through the electrolyte bath.
2. Description of the Related Art
Such substrates are used to produce pre-sensitized printing plates wherein the material comprising the substrates, which are processed in plate or strip form, is a metal, especially aluminum. Roughening of aluminum strips, for instance, for producing printing plates is done mechanically, chemically, or electrochemically, or by a combination of these roughening methods. The goal for the aluminum surface, which is used for carrying water and for the adhesion of a light-sensitive layer, is to have a certain structure and uniformity. In mechanical roughening, the surface structures have pyramid-like shapes and have different orientations longitudinally and transversely (anisotropy), while electrochemically roughened aluminum surfaces have a sponge-like structure with many tiny wells and indentations of uniform geometry longitudinally and transversely (isotropy).
In the roughening of a printing plate substrate in an electrolyte by means of alternating current in a continuous process, crosswise striations occur, also known as cross streaks, especially at low frequencies and high web speeds.
These cross streaks occur at the entry or shortly before the entry of the printing plate substrate into the operative range of the first alternating current electrode. The surface of the unroughened printing plate substrate has a nonlinear quality electrically. The cause of this nonlinear quality can be coatings of both organic and inorganic material. Especially in printing plate substrates of aluminum, an aluminum oxide coating on the surface behaves in nonlinear fashion until it has worn down completely.
Not until the coatings on the surface have worn down does a method ensue in which the current density is dependent only on the voltage and not additionally on the nature of the surface.
Depending on whether a positive or negative current first flows through part of the surface, the resistance of the surface of the printing plate substrate decreases. If some of the surface has a low resistance, then the current preferentially flows through this portion of the surface and not through the portion of the surface that has higher resistance. The higher current then causes a further decrease in the resistance. This decrease is greater than the decrease in the resistance at those points of the surface through which a lesser current flows. The differences in resistance of the surface are thus amplified still further.
Because of the variable distribution of the current density over the cross-sectional area and the change in current density as a function of the surface resistance of the substrate, electrical cross streaks develop on the substrate that are visible in the form of striations. These striations correspond to the current distribution, which is predetermined by the form of the inlet electrode. The cross streaks occur at the cycle of the alternating or three-phase current that is applied to the electrodes, depending on whether a positive or a negative half-wave arrives first.
The cross streaks, also generally known as crosswise striations or current striations, impair the visual appearance of the product, and if they are especially sharply pronounced, its quality as well. The development of these crosswise striations or current striations increases if there is high current density in the electrolyte bath at the beginning of the electrochemical roughening. The electrical behavior of the printing plate substrate and of the electrolyte is not linear at the onset of roughening, as has already been mentioned above, and varies as the roughening progresses. The loss of uniformity in the visual appearance and the loss of print quality, if the cross streaks are especially pronounced, is highly disadvantageous in the duplication of high-resolution images.
Various methods are known to avoid these cross streaks. The surface of the printing plate substrate can be provided with an additional layer, as described in German Patent DE 38 42 454 C2. By means of this additional layer, nonuniformities in the material that essentially cause spots are compensated for. The formation of crosswise striations is indeed also attenuated by this provision, but the cause of the crosswise striations is still not eliminated since this cause resides in the steep increase in current density at the entry of the substrate into the operative region of the alternating current electrode. Even with uniform coating, the crosswise striations develop, depending on whether the positive or negative half-wave of the alternating current flows first. The expense for technical equipment is high, since it is usually necessary to apply an oxide layer in an additional electrolyte. It is true that to reduce the expense the same electrolyte can be used, but the electrolytes suitable for the roughening are typically not suited, or only have limited suitability, for oxidation or for the application of other layers.
In German patent DE 39 10 450 C2, a method for producing a printing plate substrate is described in which the surface of the printing plate substrate is roughened electrochemically in an acid electrolyte using an alternating current at a frequency of 80-100 Hz, and in which the ratio of anode time to period time is from 0.25 to 0.20. Such a method requires a major expenditure for circuitry because of the high power converted, and also presents problems in the distribution of the current to the individual electrodes.
Roughening using alternating current of high, variable frequency, as described in German patent disclosure DE 39 10 213 A1, leads to a decrease in the intensity of the crosswise striations, but requires major expense for electrical equipment and limits the frequency range of the alternating current that can be utilized for optimal shaping of the surface of the printing plate substrate.
Roughening of the printing plate substrate at certain transport speeds, as have been proposed in European patent EP 0 585 586 B1, does furnish a constant imposition of equal-sized positive and negative half-waves of the alternating current on each part of the printing plate substrate, but does not take into account the fact that crosswise striations are formed essentially by the incident half-waves upon entry into the zone of the alternating current roughening.
The known methods and apparatuses do take into account or reduce the development of crosswise striations during the complete passage of the printing plate substrate through the alternating current roughening zone, but they do not prevent crosswise striations from developing at the entry of the printing plate substrate into the operative region of the alternating or three-phase current electrodes, since the current density, or the current per unit of surface area on the printing plate substrate, is of variable magnitude.
The invention provides a method for electrochemically roughening a substrate for light-sensitive layers, which comprises electrochemically roughening the surface of a metal substrate in an aqueous electrolyte bath by the application of an alternating or three-phase current to electrodes opposite the substrate while the substrate is transported continuously in a transport direction through the electrolyte bath in a roughening zone between the substrate and the first alternating or three-phase current electrode, wherein at an entry point of the substrate into the roughening zone, the current density in the electrolyte bath between the first alternating or three-phase current electrode and the substrate is less than a maximum current density for the roughening, and that the current density between the substrate and the first alternating or three-phase current electrode increases continuously up to the maximum current density with increasing distance from the entry point.
The invention also provides an apparatus for electrochemically roughening a substrate for light-sensitive layers which comprises an alternating or three-phase current electrode disposed in an electrolyte bath, which electrode has a rounded outline such that its spacing from a substrate when the substrate is passed through the electrolyte bath is greater at an entry point into a roughening zone of the electrolyte bath between the substrate and the first alternating or three-phase current electrode than within the roughening zone, and that beyond a predetermined distance from the entry point the spacing of the alternating or three-phase current electrode from the substrate is constant.
The invention further provides an apparatus for electrochemically roughening a substrate for light-sensitive layers which comprises a first alternating or three phase current electrode which spaced from a substrate in an electrolyte bath, which electrode is subdivided into electrode portions which are insulated from one another, wherein each of the electrode portions is connected to an electrical component each of which has an ohmic resistance and/or inductive and capacitive resistances, and each component of each electrode portion is connected in series with or parallel to the other components of the other electrode portions and is connected with them to an alternating or three-phase current source.
The invention further provides an apparatus for electrochemically roughening a substrate for light-sensitive layers which comprises pairs of perforated elements disposed between a first alternating or three-phase current electrode and a substrate in an electrolyte bath wherein the pairs of elements are spaced apart from one another, and wherein the elements of each pair are displaceable relative to one another.
It is accordingly the object of the present invention to improve a method of the type defined at the outset and an apparatus for performing the method in such a way that the formation of cross streaks is prevented or minimized.
This object is attained in such a way that at an entry point of the substrate into the roughening zone, the current density in the electrolyte between a first alternating or three-phase current electrode and the substrate is less than a maximum current density for the roughening, and that the current density increases continuously up to the maximum current density with increasing distance from the entry point within the region of the first alternating or three-phase current electrode.
In a feature of the invention, the increase in the current density in the electrolyte bath over the course of one period of the alternating or three-phase current amounts to less than 20% of the maximum current density.
An apparatus for performing the method is characterized in that an alternating or three-phase current electrode disposed in the electrolyte bath is curved in such a way that its spacing from a substrate passed through the electrolyte bath is greater at an entry point into a roughening zone of the electrolyte bath than within the roughening zone, and that beyond a predetermined distance from the entry point, the spacing of the alternating or three-phase current electrode from the substrate is constant.
In a further feature of the apparatus, the curved outline of the alternating or three-phase current electrode has a parabolic portion which is adjoined by a straight portion.
In a further feature of the apparatus, the first alternating or three-phase current electrode is subdivided into electrode portions, and the individual electrode portions comprise materials of different electrical conductivities. Expediently, insulating plates are disposed between the electrode portions and a further alternating or three-phase current electrode.
With the invention, the advantage is attained that by the shaping, the selection of material, and/or the different total resistances comprising ohmic and/or inductive and capacitive resistances of the alternating or three-phase current electrode, the increase in current density over the course of one period of the alternating or three-phase current is less than or equal to 20% of the maximum current density, so that cross streaks cannot occur, or only a very limited development of cross streaks occurs.
The invention will be described in further detail below in conjunction with exemplary embodiments shown in the drawings.